Acousto-optic device and fabrication method of acousto-optic device

ABSTRACT

An acousto-optic device is disclosed which can achieve both of suppression of secular change and prevention of device damage and can maintain a stabilized wavelength selection filtering operation. The acousto-optic device includes a substrate on which a SAW guide for guiding a surface acoustic wave and an optical waveguide for guiding an optical wave are provided. A pyro-electricity coping conductive wiring pattern is provided in a region of the substrate other than the SAW guide and neutralizes polarization charge generated on a surface of the substrate.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates typically to an acousto-optic device and afabrication method of an acousto-optic device, and more particularly toan acousto-optic device suitable for use with an acousto-optic tunablefilter in a reception section of a wavelength division multiplexing(WDM) transmission node for demultiplexing a signal light having adesired wavelength from wavelength division multiplexed light (WDMlight) including a plurality of multiplexed signal lights individuallyhaving different wavelengths and to a fabrication method of theacousto-optic device.

2) Description of the Related Art

In recent years, in order to achieve expansion of the transmissioncapacity and increase of the transmission distance, a WDM transmissionsystem has been popularized. In an add/drop node (optical transmissionapparatus) provided in a network which forms the WDM transmissionsystem, a dropping (branching) process and an adding (inserting) processare performed for an inputted WDM light. For example, a plurality ofsignal lights of different channels included in the inputted WDM lightare dropped and signal lights having desired wavelengths from among thewavelengths included in the local signal lights are selected and addedand outputted to a WDM transmission line.

FIG. 17 is a diagrammatic view illustrating an add/drop (Add and Drop)function. An add/drop node 90 shown in FIG. 17 selects and drops one ormore single wavelength lights (single lights) included in a WDM lightfrom a repeating node 90 a, and transmits the single lights to anoptical network 90 d. Further, the add/drop node 90 passes those singlewavelength lights which are not to be dropped to a repeating node 90 btherethrough, and adds one or more single wavelength lights from anoptical network 90 c to free wavelengths of the WDM light. This addingand dropping process is performed by an acousto-optic tunable filter(AOTF) 89 provided in the add/drop node 90.

The acousto-optic tunable filter 89 is an acousto-optic device andincludes an input port 88 a to which WDM light is to be inputted, an addport 88 b to which local signal lights are to be inputted, anon-selection port 88 c for outputting single wavelength lights whichare not dropped, and a selection port 88 d for outputting dropped singlewavelength lights.

Consequently, the acousto-optic tunable filter 89 functions as avariable wavelength filter having both functions of a band-block filterand a band-pass filter. Further, this function is implemented in onechip, and is given attention also from the point of view of the cost.

As a material for a substrate of the acousto-optic tunable filter 89,LiNbO₃ (lithium niobate) is commonly used. However, in polar crystal ofLiNbO₃ or the like, electric charge (polarization electrode) byspontaneous polarization is generated by a temperature variation orcrystal distortion (in a normal state, such spontaneous polarization isneutralized by space charge).

Then, when a TE (Transverse Electric) wave and a TM (TransverseMagnetic) wave propagate along the surface of the substrate and theinside of the substrate spaced a little from the surface of thesubstrate, the TE wave is converted into a TM wave while the TM wave isconverted into a TE wave.

Conventionally, methods for miniaturization, integration, reduction ofthe power consumption and the cost, improvement of the stability ofoperation of an acousto-optic tunable filter which can exhibit theadd/drop function have been proposed (refer to Patent Documents 1 and2).

A waveguide type optical device disclosed in the Patent Document 1includes a substrate having an electro-optical effect, an opticalwavelength, a dielectric layer on the optical waveguide, electrodes, anda conductive film layer which covers an exposed surface of thedielectric layer. Further, in the waveguide type optical device,electrical connection by the conductive film layer disposed betweenelectrodes adjacent each other is cut at least along the direction ofthe waveguide, and part of the dielectric layer is exposed at the cutportion. This stabilizes the electrode characteristic of the waveguidetype optical device and promotes the reliability thereof.

An electro-optic device disclosed in the Patent Document 2 includes aLiNbO₃ substrate including an optical waveguide formed on the upper sidesurface thereof, a buffer layer including a BCB dielectric materialdirectly formed on the upper side surface of the LiNbO₃ substrate andhaving a thickness greater than 7000 angstrom, and an electrode formedon the upper side surface of the buffer layer, arranged to allow theelectrode to receive an RF signal (Radio Frequency signal), so that theRF signal introduces an electric field to the optical waveguide.Consequently, an optical device having an improved temperature stabilityand including a discharging path for moving charge can be obtained, andformation of a strong electric field along the +z plane and the −z planeof z-cut LiNbO₃ crystal can be prevented.

Patent Document 1 Japanese Patent Laid-Open No. HEI 8-54589

Patent Document 2 Published Japanese Translation of a PCT Application,No. 2002-542511

However, charge generated in the z direction of the acousto-optictunable filter has an influence on the refractive index of thesubstrate, and makes a factor to vary the filter characteristic (bandpass characteristic) of the acousto-optic tunable filter.

Further, if electric charge generated by spontaneous polarization isaccumulated in the electrodes (comb type electrode) provided forexciting a surface acoustic wave and is discharged between theelectrodes, then there is the possibility that the electrodes or thesubstrate may be damaged.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an acousto-opticdevice and a fabrication method for the acousto-optic device which canachieve both of suppression of variation in characteristic andprevention of device damage, for example, in an optical device whichutilizes an acousto-optic effect such as an acousto-optic tunable filterfor WDM light, and can maintain a stabilized filtering operation.

(1) In order to attain the object described above, according to anaspect of the present invention, there is provided an acousto-opticdevice comprising a substrate on which a surface acoustic wavepropagation region for guiding a surface acoustic wave and an opticalwaveguide for guiding an optical wave are provided, and a conductiveneutralization wiring pattern provided in a region of the substrateother than the surface acoustic wave propagation region for neutralizingpolarization charge generated on a surface of the substrate.

With the acousto-optic device, since it comprises a conductiveneutralization wiring pattern provided in a region other than a surfaceacoustic wave propagation region for neutralizing polarization chargegenerated on a surface of the substrate, polarization charge generatedby temperature variation or crystal distortion can be neutralized by thewiring pattern formed on the surface of the substrate. Consequently,characteristic degradation of the device can be prevented and astabilized operation can be achieved.

(3) The acousto-optic device may be configured such that the substratehas a plurality of sets each including the surface acoustic wavepropagation region and the optical waveguide, and the neutralizationwiring pattern is formed so as to surround each of the surface acousticwave propagation regions, portions of the neutralization wiring patternswhich is formed between adjacent ones of the surface acoustic wavepropagation regions being formed as a common portion shared along oneline.

(4) The neutralization wiring pattern may be grounded and connected toone of electrodes for producing the surface acoustic wave or may beformed so as to surround the substrate.

(5) The substrate may include a first polarizing beam splitter fordemultiplexing a wavelength division multiplexed light in which aplurality of wavelengths are multiplexed into a plurality of opticalwaves having polarization planes different from each other, a pluralityof optical waveguides for individually propagating the plural opticalwaves demultiplexed by the first polarizing beam splitter, and a secondpolarizing beam splitter for selecting an optical wave having a desiredwavelength based on the polarization planes of the plural optical wavespropagated along the plural optical waveguides.

(6) The substrate may further include an excitation electrode forexciting the surface acoustic wave. In this instance, a buffer layer forpreventing propagation loss of the optical waves in the excitationelectrode may be formed between the substrate and the excitationelectrode. Or, the substrate may further include an absorber forabsorbing the surface acoustic wave propagated in the surface acousticwave propagation region.

(7) The substrate may include an input port for receiving a wavelengthdivision multiplexed light in which a plurality of single wavelengthlights are multiplexed, a selection port for selecting and outputtingone or more single wavelength lights from among the plural wavelengthsincluded in the wavelength division multiplexed light inputted from theinput port, and a non-selection port for outputting one or more singlewavelength lights which are not selected by the selection port.

With the acousto-optic device of the configuration described, both ofsuppression of characteristic variation and prevention of device damagecan be achieved, for example, in an optical device which utilizes anacousto-optic effect such as an acousto-optic tunable filter for WDMlight or the like.

(8) According to another aspect of the present invention, there isprovided a fabrication method of an acousto-optic device for forming anacousto-optic device including a substrate on which a surface acousticwave propagation region for guiding a surface acoustic wave and anoptical waveguide for guiding an optical wave are provided on a wafersubstrate, comprising an excitation electrode forming step of forming anexcitation electrode for exciting the surface acoustic wave on the wafersubstrate, and a neutralization wiring pattern forming step of forming aconductive neutralization wiring pattern provided in a region of thesubstrate other than the surface acoustic wave propagation region forneutralizing polarization charge generated on a surface of thesubstrate.

With the fabrication method, since the neutralization wiring pattern isformed on the surface of the substrate, polarization charge generated bytemperature variation or crystal distortion can be neutralized. Further,characteristic degradation of the device can be suppressed and astabilized operation can be achieved.

(8) The neutralization wiring pattern forming step may include a secondneutralization wiring pattern forming step of forming, individually fora plurality of such substrates, the neutralization wiring pattern in aregion of the substrate other than the surface acoustic wave propagationregion.

(9) The fabrication method may further comprise a vapor deposition stepof vapor depositing conductive substance on a surface of the substrate,and a diffusion step of diffusing the conductive material vapordeposited at the vapor deposition step into the inside of the substrate.

(10) In this instance, the fabrication method may further comprise abuffer layer forming step of forming a pattern of a buffer layer forpreventing propagation loss of the optical waves in the excitationelectrode between the substrate and the excitation electrode.

(11) The fabrication method may further comprise a surface acoustic wavepropagation region forming step of forming the surface acoustic wavepropagation region.

(12) The fabrication method may further comprise a neutralization wiringpattern grounding step of grounding the neutralization wiring patternformed at the neutralization wiring pattern forming step.

(13) In this instance, the fabrication method may further comprise aconnection step of connecting the neutralization wiring pattern groundedat the neutralization wiring pattern grounding step and the excitationelectrode formed at the excitation electrode forming step to each other.

(14) The fabrication method may further comprise a second neutralizationwiring pattern forming step of forming the neutralization wiring patternon a face different from that face of the surface of the substrate onwhich the surface acoustic wave propagation region is provided.

(15) The fabrication method may further comprise a thin film formingstep of forming a conductive thin film on a face different from thatface of the surface of the substrate on which the surface acoustic wavepropagation region is provided.

With the fabrication method, a stabilized filtering operation can bemaintained.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description and theappended claims, taken in conjunction with the accompanying drawings inwhich like parts or elements are denoted by like reference characters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of an acousto-optic tunable filter to whichthe present invention is applied;

FIG. 2A is a perspective view of an acousto-optic tunable filteraccording to an embodiment of the present invention;

FIG. 2B is a sectional view of a LiNbO₃ substrate taken along line A-A′of FIG. 2A;

FIG. 3 is a schematic view showing a comb type electrode according tothe embodiment of the present invention;

FIG. 4A is a diagrammatic view illustrating an input/outputcharacteristic where a wavelength is selected according to theembodiment of the present invention;

FIG. 4B is a diagrammatic view illustrating an input/outputcharacteristic where no wavelength is selected according to theembodiment of the present invention;

FIG. 5 is a diagrammatic view showing an example of a configuration of amodification to the acousto-optic tunable filter according to theembodiment of the present invention;

FIGS. 6A to 6J are schematic views illustrating fabrication steps of theacousto-optic tunable filter according to the embodiment of the presentinvention;

FIGS. 7A to 7F are schematic views illustrating fabrication steps wherea plurality of pyro-electricity coping wiring patterns are formedaccording to the embodiment of the present invention;

FIG. 8 is a top plan view of a chip fabricated through the fabricationsteps shown in FIGS. 7A to 7F;

FIGS. 9A to 9D are diagrammatic views illustrating different fabricationsteps where a plurality of pyro-electricity coping wiring patterns areformed according to the embodiment of the present invention;

FIG. 10 is a schematic view showing a third wiring pattern according tothe embodiment of the present invention;

FIG. 11 is a schematic view showing a fourth wiring pattern according tothe embodiment of the present invention;

FIG. 12 is a schematic view showing a fifth wiring pattern according tothe embodiment of the present invention;

FIG. 13 is a schematic view showing a sixth wiring pattern according tothe embodiment of the present invention;

FIG. 14A is a schematic view showing a seventh wiring pattern accordingto the embodiment of the present invention;

FIG. 14B is a sectional view of the substrate taken along line A-A′ ofFIG. 14A;

FIG. 15 is a schematic view showing an eighth wiring pattern accordingto the embodiment of the present invention;

FIG. 16A is a schematic view showing a ninth wiring pattern according tothe embodiment of the present invention;

FIG. 16B is a sectional view of the substrate taken along line A-A′ ofFIG. 16A; and

FIG. 17 is schematic view illustrating an add/drop function.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following, an embodiment of the present invention is describedwith reference to the drawings.

(A) Description of an Embodiment of the Present Invention

FIG. 1 is a top plan view of an acousto-optic tunable filter (AOTF) towhich the present invention is applied. Referring to FIG. 1, theacousto-optic tunable filter 1 shown includes a substrate 10 on which aSAW (Surface Acoustic Wave) guide (surface acoustic wave propagationregion) 2 for guiding a surface acoustic wave and optical waveguides 11a, 11 b, 11 c, 11 d, 11 e and 11 f for guiding optical waves (forexample, inputted WDM light) are provided. The acousto-optic tunablefilter 1 functions as a wavelength selection filter for filtering anoptical wave having a desired wavelength using an acousto-optic effectbetween a surface acoustic wave and an optical wave.

As hereinafter described, the acousto-optic tunable filter 1 includes apyro-electricity coping conductive wiring pattern (neutralization wiringpattern) 14 for neutralizing polarization charge generated on thesurface of the substrate 10. The conductive wiring pattern(neutralization wiring pattern) 14 is provided in a region differentfrom a region in which the SAW guide 2 is provided, for example, in sucha manner as to surround the SAW guide 2 formed from a thin film or thelike as hereinafter described. Further, on an upper face of thesubstrate 10 formed from a piezoelectric substrate of LiNbO₃ (lithiumniobate) or the like, the optical waveguides 11 a, 11 b, 11 c, 11 d, 11e and 11 f are formed, and an input side polarization beam splitter(PBS) 12 a, an output side polarization beam splitter 12 b, and a combtype electrode (Inter Digital Transducer: IDT or excitation electrode)13 are provided. Further, as hereinafter described with reference toFIG. 3, the acousto-optic tunable filter 1 is connected to an RF signaloscillator (high-frequency signal oscillator) 50.

It is to be noted that, for example, as shown in FIG. 3, the “upperface” of the substrate 10 signifies a face on the upper side of thesubstrate 10, and the “surface” of the substrate 10 signifies a surfacelayer of the substrate 10.

(1) Substrate 10

The substrate 10 is formed by thermal diffusion of titanium Ti into aLiNbO₃ substrate having a birefringence property of light. It is to benoted that a substrate formed from a material having a piezoelectriceffect different other than LiNbO₃ may be used.

FIG. 2A is a perspective view of the acousto-optic tunable filter 1according to the first embodiment of the present invention which is inan x-cut state. The x, y and z directions shown in FIG. 2A are athicknesswise direction of the substrate 10, an advancing direction ofpassing light, and a depthwise direction of the substrate 10,respectively. Further, a voltage application direction is the zdirection. Here, if a voltage is applied in the x direction to thesubstrate 10, then charge is generated in the +z and −z directions by apyro-electric effect, and the substrate 10 is polarized in the +z and −zdirections.

A layer structure of the substrate 10 along line A-A′ in FIG. 2A isshown in FIG. 2B. The SAW guide 2 shown in FIG. 2B is formed from a thinfilm as hereinafter described with reference to FIGS. 6C and 6H (thestructure of the SAW guide 2 of the present invention is not limited tothis particular structure). It is to be noted that, preferably, forportions at which the pyro-electricity coping wiring pattern 14 shown inFIG. 2B intersects with the optical waveguides 11 a, 11 b, 11 e and 11f, a buffer layer (not shown) is provided under the pyro-electricitycoping wiring pattern 14.

(2) Upper Face of the Substrate 10

The optical waveguides 11 a to 11 f, input side polarization beamsplitter 12 a and output side polarization beam splitter 12 b are formedon the upper face of the substrate 10 shown in FIG. 2A.

(2-1) Optical Waveguides 11 a to 11 f

(i) The optical waveguides 11 a and 11 b are provided in order topropagate two optical waves demultiplexed by the input side polarizationbeam splitter 12 a, and are connected to an input port 88 a forinputting WDM light and an add port 88 b for inputting add light (whichis light to be added to the WDM light), respectively.

(ii) The optical waveguides 11 c and 11 d function as TE-TM modeconversion sections by an interaction with a surface acoustic wavehereinafter described.

(iii) The optical waveguides 11 e and 11 f are connected to anon-selection port 88 c and a selection port 88 d, and output a selectedsignal light and a non-selected signal light from within inputted WDMlight, respectively. The selection port 88 d selects and outputs one ormore single wavelength lights from among n wavelengths included in theWDM light inputted from the input port 88 a. The non-selection port 88 coutputs one or more single wavelength lights which are not selected bythe selection port 88 d.

(2-2) Input Side Polarization Beam Splitter (First Polarization BeamSplitter) 12 a and Output Side Polarization Beam Splitter (SecondPolarization Beam Splitter) 12 b

The input side polarization beam splitter 12 a demultiplexes WDM lightincluding n wavelengths multiplexed with each other into two opticalwaves having polarization planes different from each other and branchesthe demultiplexed optical waves towards the optical waveguides 11 c and11 d. Meanwhile, the output side polarization beam splitter 12 b selectsboth or one of optical waves propagated in the optical waveguides 11 cand 11 d based on the polarization planes of the optical waves.Consequently, the two optical waves of the TE wave and TM wave arepropagated in the optical waveguides 11 c and 11 d, and the polarizationplanes of the TE wave and TM wave are rotated in a mode conversionregion (a region in which an optical wave having a specific wavelengthand a surface acoustic wave interact with each other) to convert themodes of the TE wave and TM wave. Passing lights from between the modeconverted optical waves are selected, and the selected lights and theother non-selected lights are branched and outputted from the selectionport 88 d and the non-selection port 88 c, respectively.

(2-3) Excitation of Surface Acoustic Waves in the Optical Waveguides 11c and 11 d

FIG. 3 is a schematic view showing the comb type electrode 13 accordingto the embodiment of the present invention. In FIG. 3, like elements tothose described above are denoted by like reference characters.

Here, the RF signal oscillator 50 oscillates, for example, for a signallight of approximately 1.5 μm, a controlling high-frequency signal (forexample, RF signal, sinusoidal wave or sine wave) of 170 MHz. Further,by variation of the oscillation frequency, a wavelength of a signallight to be filtered is selected.

(2-4) Comb Type Electrode (IDT) 13

The comb type electrode 13 excites a surface acoustic wave using thecontrolling RF signal oscillated by the RF signal oscillator 50, andincludes, as an example of a shape of the electrode, electrodes 13 a and13 b for inputting and outputting the RF signal and electrode fingers 13c to 13 f having a period substantially equal to the wavelength of thesurface acoustic wave to be excited. Further, the comb type electrode 13is connected to a switch 69 for changing over the RF signal betweenconnection to and disconnection from the comb type electrode 13. If theRF signal is applied between the input and output electrodes 13 a and 13b, then crystal distortion is generated in the inside of the substrate10 by a piezoelectric effect, and the crystal distortion propagates as asurface acoustic wave in a longitudinal direction (FIG. 1 and so forth)of the substrate 10.

(2-5) Wavelength Selection Function of the Acousto-optic Tunable Filter

FIG. 4A is a diagrammatic view illustrating an input/outputcharacteristic where a wavelength is selected according to theembodiment of the present invention, and, in FIGS. 4A and 4B, likeelements to those described above are denoted by like referencecharacters. Further, an intensity characteristic (input characteristic)of signal light is a fixed intensity within a range, for example, from1,530 nm to 1,610 nm. Here, where the switch 69 shown in FIG. 4A is on,a TE wave and a TM wave of a signal light having, for example, awavelength of 1545 nm are rotated and converted into a TM wave and a TEwave by the optical waveguides 11 c and 11 d, respectively. Further, alight having the wavelength of 1545 nm included in the TE wave and theTM wave is selected by the output side polarization beam splitter (notshown), and the selected light is outputted from the selection port 88d. Meanwhile, non-selected lights other than the rotated signal lightare outputted from the non-selection port 88 c (FIG. 1). Consequently,as shown from an intensity characteristic (output characteristic) ofsignal light, a function as a band-pass filter is achieved.

Meanwhile, FIG. 4B is a diagrammatic view illustrating an input/outputcharacteristic where no wavelength is selected according to theembodiment of the present invention. Where the switch 69 shown in FIG.4B is off, the selection light does not appear in the outputcharacteristics of the TE wave and TM wave.

In this manner, wavelength division multiplexed signal light of n wavescan be demultiplexed into the selection light and the non-selectionlights.

(2-6) Modification

It is to be noted that FIG. 5 is a schematic view showing aconfiguration of a modification to the acousto-optic tunable filteraccording to the embodiment of the present invention. Referring to FIG.5, the acousto-optic tunable filter 21 shown is configured such that thecomb type electrode 13 is inclined with respect to a center line in alongitudinal direction of the substrate 10, and a SAW guide 2 a isformed in a direction same as an advancing direction of a surfaceacoustic wave excited by the comb type electrode 13.

(3) Pyro-electricity Coping Wiring Pattern 14 (Neutralization WiringPattern of the Acousto-optic Tunable Filter 1)

A pyro-electricity coping wiring pattern 14 is formed so as to connectboth ends in the z direction (polarization direction) of the upper faceof the substrate 10 to each other, and is formed along an outerperiphery of the upper face, on which the SAW guide 2 is provided, fromwithin the surface of the substrate 10 such that a region wherein anacousto-optic effect is generated is surrounded. Further, since thepyro-electricity coping wiring pattern 14 is electrically closed, chargegenerated on the upper face of the substrate is neutralized.

In this manner, with the present acousto-optic tunable filter 1,polarization charge generated on the upper face of the substrate 10formed from LiNbO₃ can be neutralized, and therefore, a stabilizedwavelength selection function can be exhibited.

(4) Fabrication Steps of the Acousto-optic Tunable Filter 1

Next, a fabrication method of the acousto-optic tunable filter 1 isdescribed in detail with reference to FIGS. 6A to 6J.

FIGS. 6A to 6J are schematic views illustrating fabrication steps of theacousto-optic tunable filter according to the embodiment of the presentinvention. In particular, FIGS. 6A to 6E are top plan views of thesubstrate 10. FIGS. 6F to 6H are sectional views of the substrate 10taken along line B-B′ of FIGS. 6A to 6C, respectively. Further, FIGS. 6Iand 6J are sectional views of the substrate 10 taken along line C-C′ ofFIGS. 6D and 6E, respectively.

(i) At a first step, titanium Ti is vapor deposited on the upper face ofthe substrate 10 (FIG. 2A and so forth) to perform patterning for theoptical waveguides 11 a to 11 f (FIG. 6A) and form the pyro-electricitycoping wiring pattern 14 on the upper face of the substrate 10 (FIG.6F).

(ii) At a next step, titanium Ti is diffused into the inside of thesubstrate 10 (FIG. 6G).

(iii) At a further step, silica (silicon dioxide) SiO₂ doped with indiumIn is deposited at a portion which is to be formed as the modeconversion region, and patterning for the SAW guide 2 for guiding asurface acoustic wave is performed (FIGS. 6C and 6H).

(iv) At a succeeding step, silica SiO₂ is deposited at both of an inputside end potion and an output side end portion (in the longitudinaldirection) of the substrate 10 to perform patterning for the bufferlayer (not shown) (FIGS. 6D and 6I).

Accordingly, in the fabrication method of the present acousto-optictunable filter 1, the pattern for the buffer layer for preventingpropagation loss of an optical wave in the comb type electrode 13 isformed between the substrate 10 and the comb type electrode 13 (bufferlayer forming step).

(v) Then, through vapor deposition of gold Au, patterning of wiringlines for both of the comb type electrode 13 and the pyro-electricitycoping wiring pattern 14 is performed (FIGS. 6E and 6J).

The comb type electrode 13 and the pyro-electricity coping wiringpattern 14 are formed in this manner. Here, in FIG. 6J, referencecharacters L1 and L2 denote a vapor deposited Au layer and the SAW guide2 on which silica SiO₂ doped with In is deposited, respectively, and theoptical waveguides 11 c and 11 d are formed in the substrate 10.

In summary, in the fabrication method of the present acousto-optictunable filter 1, the acousto-optic tunable filter 1 as an acousto-opticdevice having the substrate 10 on which the SAW guide 2 for guiding asurface acoustic wave and the optical waveguides 11 c and 11 d forguiding optical waves are provided is formed on a wafer (not shown).

First, the comb type electrode 13 for exciting a surface acoustic waveis formed on the substrate 10 of the wafer (excitation electrode formingstep).

Then, the conductive pyro-electricity coping wiring pattern 14 providedin a region of the substrate other than the region where the SAW guide 2is provided for neutralizing polarization charge generated on thesurface of the substrate is formed (neutralization wiring patternforming step).

Thereafter, a conductive material is vapor deposited on the surface ofthe substrate 10 (vapor deposition step) and the conductive materialvapor deposited at the vapor deposition step is diffused into the insideof the substrate (diffusion step), and the SAW guide 2 is formed(surface acoustic wave propagation region forming step).

The comb type electrode 13 and the pyro-electricity coping wiringpattern 14 are formed in this manner.

Next, a fabrication method of a device wherein a plurality of (forexample, 3) pyro-electricity coping wiring patterns 14 are formed on thesame wafer to integrate a plurality of chips is described. Here, anacousto-optic tunable filter of FIG. 8 hereinafter described is a basicacousto-optic tunable filter, and FIGS. 7A to 7F and 9A to 9Dindividually indicate arrangements wherein a groove is provided betweenadjacent acousto-optic tunable filters as a countermeasure againstleakage of a surface acoustic wave.

FIGS. 7A to 7F are schematic views illustrating fabrication steps where,for example, 3 pyro-electricity coping wiring patterns 14 according tothe embodiment of the present invention are formed. A substrate 10 ashown in FIGS. 7A to 7F has three acousto-optic tunable filters 1 formedon one wafer. Here, FIGS. 7A to 7C are top plan views of the wafer, andFIGS. 7D to 7F are sectional views of the wafer shown in FIGS. 7A to 7Ctaken along line D-D′, respectively. Further, in the followingfabrication steps, it is assumed that the formation of thepyro-electricity coping wiring patterns 14 is started in a state whereinthe comb type electrode 13 is formed in FIGS. 7A to 7C.

(i) At a first step, patterning of the SAW guide 2 is performed (FIGS.7A and 7D).

(ii) At a next step, grooves G1 and G2 are formed on the substrate 10 ausing a cutting saw or by etching (FIGS. 7B and 7E). Naturally, here,the grooves G1 and G2 need not necessarily be formed in the fabricationprocess.

(iii) At a further step, gold Au is vapor deposited in the grooves G1and G2 to form the pyro-electricity coping wiring patterns 14 (FIGS. 7Cand 7F).

Or else, in FIGS. 7A to 7F, two or four or more pyro-electricity copingwiring patterns 14 may be formed. Since the fabrication steps of the twoor four or more pyro-electricity coping wiring patterns 14 are same asthose in the formation of the three pyro-electricity coping wiringpatterns 14, overlapping description thereof is omitted herein.

FIG. 8 is a top plan view of the chip fabricated through the fabricationsteps illustrated in FIGS. 7A to 7F. Referring to FIG. 8, the substrate10 a shown has three substrates 10, and the pyro-electricity copingwiring patterns (neutralization wiring patterns) 14 are formed so as tosurround the three substrates 10. In particular, three acousto-optictunable filters are fabricated on the wafer (not shown) in a statewherein they are surrounded by the pyro-electricity coping wiringpatterns 14. While the pyro-electricity coping wiring patterns 14 in theforegoing description are formed in the grooves, the pyro-electricitycoping wiring patterns 14 may be formed otherwise without providing thegrooves.

Accordingly, the acousto-optic tunable filter according to the presentfabrication method has three sets of the SAW guide 2 and opticalwaveguides 11 a, 11 b, 11 c, 11 d, 11 e and 11 f. The pyro-electricitycoping wiring patterns 14 are formed so as to surround the individualSAW guides 2, and a pattern portion formed between the SAW guides 2 canbe configured so as to be shared along one line.

Further, since the pyro-electricity coping wiring patterns 14 are formedusing steps same as the forming steps of the wiring pattern for the combtype electrode 13, although the pyro-electric countermeasure is taken,the number of steps does not increase.

In this manner, at the pyro-electric-coping wiring pattern forming step,the pyro-electricity coping wiring patterns 14 are formed, for each ofthe three substrates 10, in a region other than the region wherein theSAW guide 2 is provided (second neutralization wiring pattern formingstep).

FIGS. 9A to 9D are schematic views illustrating different fabricationsteps where, for example, three pyro-electricity coping wiring patterns14 according to the embodiment of the present invention are formed. Inparticular, FIGS. 9A and 9B are top plan views of a wafer, and FIGS. 9Cand 9D are sectional views of the substrate 10 a shown in FIGS. 9A and9B taken along line E-E′, respectively.

(i) At a first step, gold Au is vapor deposited and patterning of thecomb type electrode 13 and the pyro-electricity coping wiring patterns14 is performed (FIG. 9A). Here, the gold Au remains on an upper face ofthe substrate 10 a (FIG. 9C).

(ii) At a next step, grooves are formed on the substrate 10 a using acutting saw or by etching (FIGS. 9B and 9D).

Further, in the FIGS. 9A to 9D, also the number of sets of the SAW guide2 and optical waveguides 11 a, 11 b, 11 c, 11 d, 11 e and 11 f mayotherwise be two or four or more. In particular, two or four or morepyro-electricity coping wiring patterns 14 may be formed. Since also theforming steps of the two or four or more pyro-electricity coping wiringpatterns 14 are same as the forming steps for forming threepyro-electricity coping wiring patterns 14, overlapping descriptionthereof is omitted herein.

In this manner, the grooves can be formed on the surface of thesubstrate 10 a after metal is vapor deposited on the surface of thesubstrate 10 a.

FIG. 10 is a schematic view showing a third wiring pattern according tothe embodiment of the present invention. Referring to FIG. 10, thesubstrate 10 b shown has an electrically closed pyro-electricity copingwiring pattern 14 formed thereon, and the pyro-electricity coping wiringpattern 14 is grounded. The fabrication method of the substrate 10 bshown in FIG. 10 includes steps described in (i) to (iii) below.

(i) The pyro-electricity coping wiring pattern 14 is formed along anouter periphery of the surface of the substrate 10 b in such a manner asto surround the device region.

(ii) The pyro-electricity coping wiring pattern 14 is formed inaccordance with a step same as the wiring pattern forming step of thecomb type electrode 13.

(iii) The pyro-electricity coping wiring pattern 14 formed at theneutralization wiring pattern forming step of (ii) above is grounded(neutralization wiring pattern grounding step).

In this manner, the substrate 10 b on which the pyro-electricity copingwiring pattern 14 is grounded is obtained.

FIG. 11 is a schematic view showing a fourth wiring pattern according tothe embodiment of the present invention. Referring to FIG. 11, in thesubstrate 10 c shown, the pyro-electricity coping wiring pattern 14 isgrounded and connected to one of the electrodes 13 a and 13 b of thecomb type electrode 13 for producing a surface acoustic wave. Thefabrication method of the acousto-optic tunable filter shown in FIG. 11includes steps described in (i) to (iv) below.

(i) The pyro-electricity coping wiring pattern 14 is formed along anouter periphery of the surface of the substrate 10 c in such a manner asto surround the device region.

(ii) The pyro-electricity coping wiring pattern 14 is formed inaccordance with a step same as the wiring pattern forming step of thecomb type electrode 13.

(iii) The pyro-electricity coping wiring pattern 14 is grounded.

(iv) The pyro-electricity coping wiring pattern 14 grounded at theneutralization wiring pattern grounding step of (iii) and one of theelectrodes 13 a and 13 b of the comb type electrode 13 formed at theexcitation electrode forming step are connected to each other.

Consequently, the number of man-hours in bonding of the comb typeelectrode 13 is reduced.

The pyro-electricity coping wiring pattern 14 may be formed also on therear side face (rear face) of any of the substrates 10 to 10 d.

FIG. 12 is a schematic view showing a fifth wiring pattern according tothe embodiment of the present invention. Referring to FIG. 12, thepyro-electricity coping wiring pattern 14 is formed on the rear face ofthe substrate 10, 10 c or the like so that polarization charge isneutralized through the pyro-electricity coping wiring pattern 14 on therear face. The fabrication method of the acousto-optic tunable filtershown in FIG. 12 includes steps described in (i) to (iii) below.

(i) The pyro-electricity coping wiring pattern 14 is formed along anouter periphery of the surface of the substrate 10 d in such a manner asto surround the device region.

(ii) The pyro-electricity coping wiring pattern 14 is formed inaccordance with a step same as the wiring pattern forming step of thecomb type electrode 13.

(iii) The pyro-electricity coping wiring pattern 14 is formed on therear face.

Consequently, charge is neutralized through the pyro-electricity copingwiring pattern 14 formed on the rear face.

FIG. 13 is a schematic view showing a sixth wiring pattern according tothe embodiment of the present invention. Referring to FIG. 13, thepyro-electricity coping wiring pattern 14 shown is provided on the rearface but does not extend along an outer periphery of the substrate 10 e.Accordingly, when the pyro-electricity coping wiring pattern 14 is to beformed on the rear face, it can be formed at any desired place orportion irrespective of the position of the device region. Thefabrication method of the acousto-optic tunable filter shown in FIG. 13is same as that of the acousto-optic tunable filter shown in FIG. 12,and overlapping description thereof is omitted herein.

FIG. 14A is a schematic view showing a seventh wiring pattern accordingto the embodiment of the present invention, and FIG. 14B is a sectionalview of the seventh wiring pattern according to the embodiment of thepresent invention taken along line A-A′ of FIG. 14A. Referring to FIG.14A, the pyro-electricity coping wiring pattern 14 is extended to sidefaces of the substrate 10 f. The fabrication method of the acousto-optictunable filter shown in FIGS. 14A and 14B includes steps described in(i) to (iv) below.

(i) The pyro-electricity coping wiring pattern 14 is formed along anouter periphery of the surface of the substrate 10 b in such a manner asto surround the device region.

(ii) The pyro-electricity coping wiring pattern 14 is formed inaccordance with a step same as the wiring pattern forming step of thecomb type electrode 13.

(iii) The pyro-electricity coping wiring pattern 14 is formed on therear face.

(iv) The pyro-electricity coping wiring pattern 14 is formed so as toextend to the side faces.

Accordingly, at each step illustrated in FIGS. 14A and 14B, thepyro-electricity coping wiring pattern 14 is formed on those side facesfrom within the surface of the substrate 10 f which are different fromthe upper face on which the SAW guide 2 is provided (secondneutralization wiring pattern forming step).

Consequently, a higher neutralization effect can be achieved.

FIG. 15 is a schematic view showing an eighth wiring pattern accordingto the embodiment of the present invention. Referring to FIG. 15, in theacousto-optic tunable filter shown, the conductive thin film 19 isformed on the rear face of the substrate 10 g. Further, thepyro-electricity coping wiring pattern 14 is formed on the rear face sothat charge can be removed more efficiently. The fabrication method ofthe acousto-optic tunable filter shown in FIG. 15 includes stepsdescribed in (i) to (v) below.

(i) The pyro-electricity coping wiring pattern 14 is formed along anouter periphery of the surface of the substrate 10 g in such a manner asto surround the device region.

(ii) The pyro-electricity coping wiring pattern 14 is formed inaccordance with a step same as the wiring pattern forming step of thecomb type electrode 13.

(iii) The pyro-electricity coping wiring pattern 14 is formed on therear face.

(iv) The rear face is covered with the conductive film 19. Inparticular, the conductive film 19 is formed on that face (rear face)from within the surface of the substrate 10 g which is different fromthe face (upper face) on which the SAW guide 2 is provided (thin filmforming step).

(v) The pyro-electricity coping wiring pattern 14 is formed such that itextends to the side faces.

Consequently, also the rear face of the substrate 10 g is covered withthe conductive thin film 19, and polarization charge on the surface ofthe substrate 10 g is neutralized through the conductive thin film 19.Consequently, a higher neutralization effect can be achieved.

FIG. 16A is a schematic view showing a ninth wiring pattern according tothe embodiment of the present invention, and FIG. 16B is a sectionalview of a substrate 10 h shown in FIG. 16A taken along line A-A′.

Also side faces of the substrate 10 h shown in FIG. 16B can be coveredwith the conductive thin film 19. The fabrication method of theacousto-optic tunable filter shown in FIG. 16A includes steps describedin (i) to (vi) below.

(i) The pyro-electricity coping wiring pattern 14 is formed along anouter periphery of the surface of the substrate 10 h in such a manner asto surround the device region.

(ii) The pyro-electricity coping wiring pattern 14 is formed inaccordance with a step same as the wiring pattern forming step of thecomb type electrode 13.

(iii) The pyro-electricity coping wiring pattern 14 is formed on therear face.

(iv) The rear face is covered with the conductive thin film 19.

(v) The pyro-electricity coping wiring pattern 14 is formed on the sidefaces.

(vi) The side faces are covered with the conductive thin film 19.

Accordingly, the step illustrated in FIGS. 16A and 16B forms thepyro-electricity coping wiring pattern 14 on one or both of those sidefaces and rear face from within the surface of the substrate 10 h whichare different from the upper face on which the SAW guide 2 is provided(second neutralization wiring pattern forming step). Further, this stepforms the conductive thin film 19 on one or both of those side faces andrear face from within the surface of the substrate 10 h which aredifferent from the upper face on which the SAW guide 2 is provided (thinfilm forming step) Consequently, since the conductive thin film 19 isdeposited also on the side faces of the substrate 10 h, polarizationcharge is neutralized.

In this manner, the fabrication method of the acousto-optic tunablefilter can neutralize polarization charge generated by a temperaturevariation or a pressure variation by means of the pyro-electricitycoping wiring pattern 14 formed on the surface of the substrate 10 a to10 h to suppress deterioration in characteristic of the device.

In this manner, with the present acousto-optic device 1, both ofsuppression of characteristic variation and prevention of damage to thedevice in a variable wavelength filter for WDM light can be achieved,and a stabilized filtering operation can be maintained.

(B) Others

The present invention is not limited to the embodiment and themodification specifically described above, and variations andmodifications can be made without departing from the scope of thepresent invention.

An acousto-optic tunable filter fabricated by the fabrication method ofan acousto-optic filter can be used in an add/drop node (refer to, forexample, FIG. 17) and functions as a band-block filter or a band-passfilter.

1. An acousto-optic device, comprising: a substrate on which a surfaceacoustic wave propagation region for guiding a surface acoustic wave andan optical waveguide for guiding an optical wave are provided; and aconductive neutralization wiring pattern provided in a region of saidsubstrate other than said surface acoustic wave propagation region,wherein said conductive neutralization wiring pattern comprises a firstand second pattern which are electrically connected to each other andformed on an upper or rear surface of said substrate in such a mannerthat the first and second pattern sandwich in said surface acoustic wavepropagation region from both sides with respect to a directionsubstantially perpendicular to a guiding direction of said optical wave.2. The acousto-optic device as claimed in claim 1, wherein saidsubstrate has a plurality of sets each including said surface acousticwave propagation region and said optical waveguide, and saidneutralization wiring pattern is formed so as to surround each of thesurface acoustic wave propagation regions, portions of theneutralization wiring patterns which is formed between adjacent ones ofsaid surface acoustic wave propagation regions being formed as a commonportion shared along one line.
 3. The acousto-optic device as claimed inclaim 1, wherein said neutralization wiring pattern is formed so as toconnect the opposite ends of the surface of said substrate in apolarization direction to each other.
 4. The acousto-optic device asclaimed in claim 1, wherein said neutralization wiring pattern is formedso as to surround a region in which an acousto-optic effect occurs alongan outer periphery of that face of the surface of said substrate onwhich said surface acoustic wave propagation region is provided.
 5. Theacousto-optic device as claimed in claim 1, wherein said neutralizationwiring pattern is formed so as to surround said substrate.
 6. Theacousto-optic device as claimed in claim 1, wherein said neutralizationwiring pattern is grounded and connected to one of electrodes forproducing the surface acoustic wave.
 7. The acousto-optic device asclaimed in claim 1, wherein said substrate includes: a first polarizingbeam splitter for demultiplexing a wavelength division multiplexed lightin which a plurality of wavelengths are multiplexed into a plurality ofoptical waves having polarization planes different from each other; aplurality of optical waveguides for individually propagating the pluraloptical waves demultiplexed by said first polarizing beam splitter; anda second polarizing beam splitter for selecting an optical wave having adesired wavelength based on the polarization planes of the pluraloptical waves propagated along said plural optical waveguides.
 8. Theacousto-optic device as claimed in claim 1, wherein said substratefurther includes an excitation electrode for exciting the surfaceacoustic wave, and a buffer layer for preventing propagation loss of theoptical waves in said excitation electrode is formed between saidsubstrate and said excitation electrode.
 9. The acousto-optic device asclaimed in claim 1, wherein said substrate further includes an absorberfor absorbing the surface acoustic wave propagated in the surfaceacoustic wave propagation region.
 10. The acousto-optic device asclaimed in claim 1, wherein said substrate includes: an input port forreceiving a wavelength division multiplexed light in which a pluralityof single wavelength lights are multiplexed; a selection port forselecting and outputting one or more single wavelength lights from amongthe plural wavelengths included in the wavelength division multiplexedlight inputted from said input port; and a non-selection port foroutputting one or more single wavelength lights which are not selectedby said selection port.
 11. The acousto-optic device as claimed in claim1, wherein said conductive neutralization wiring pattern furthercomprising a third pattern formed on a same surface as said first andsecond pattern and for electrically connecting said first and secondpattern.
 12. The acousto-optic device as claimed in claim 1, whereinsaid conductive neutralization wiring pattern further comprising a thirdpattern formed on a same surface as said first and second pattern andfor electrically connecting said first and second pattern.
 13. Anacousto-optic device having a substrate on which an elastic surface-wavepropagation region for guiding an elastic surface-wave and an opticalwaveguide for guiding an optical wave, comprising: a conductiveneutralization wiring pattern provided in a region of said substrateother than said elastic surface-wave propagation region, wherein saidconductive neutralization wiring pattern comprises a first and secondpattern which are electrically connected to each other and formed on anupper or rear surface of said substrate in such a manner that the firstand second pattern sandwich in said surface acoustic wave propagationregion from both sides with respect to a direction substantiallyperpendicular to a guiding direction of said optical wave.